(1) Field of the Invention
The present invention relates to a lithium rechargeable battery. More particularly, the present invention relates to an improved active material for a positive electrode in lithium rechargeable batteries.
(2) Description of the Prior Art
Recently, attention has been given to nonaqueous batteries which use, as the negative electrode active material, an alloy or a carbon material capable of absorbing and releasing lithium, and use, as the positive electrode active material, a lithium transition metal composite oxide. This is because such nonaqueous batteries have high energy density.
The general lithium transition metal composite oxides are oxides which contain Li and one or more metal elements selected from Co, Ni, Fe, Mn, and Cu. The representative examples of such lithium transition metal composite oxides are LiCoO2, LiMnO2, and LiFeO2.
Of the above lithium transition metal composite oxides, lithium cobalt oxide (LiCoO2) is typical. Although it is expected that use of lithium manganese oxide (LiMnO2) as the positive electrode active material will exceed that of lithium cobalt oxide since lithium manganese oxide includes manganese which is cheap and occurs in abundance.
Also, lithium transition metal composite oxides using manganese have higher thermal stability than those using cobalt. Accordingly, it is expected that a battery adopting a lithium transition metal composite oxide using manganese as the positive electrode active material will show superior storage characteristics in charging and superior thermal stability against overcharge.
However, use of a lithium manganese oxide as the positive electrode active material has a problem that charge/discharge characteristics, such as the discharge capacity, decrease noticeably as the number of charge/discharge cycles increases. It is generally considered that this deficiency is generated when the crystal structure of the lithium manganese oxide becomes unstable as the number of charge/discharge cycles increases.
More particularly, as the number of performed charge/discharge cycles increases, only manganese ions react and solve into the electrolyte solution, resulting in unstableness of the crystal structure of the lithium manganese oxide. This causes the absorbing and releasing ability of the lithium ions to decrease, resulting in reduction in the battery capacity.
It is therefore an object of the present invention to provide a lithium rechargeable battery having a superior battery performance, the lithium rechargeable battery using manganese, being cheap and occurring in abundance, as an element of the positive electrode active material.
The above object is achieved by a lithium rechargeable battery including a negative electrode and a positive electrode, the negative electrode being made of a material capable of absorbing and releasing lithium, and the positive electrode being made of a lithium-containing metal oxide capable of absorbing and releasing lithium, wherein the lithium-containing metal oxide is represented by a compositional formula LixMn1xe2x88x92yxe2x88x92zNiyFezO2, wherein 1xe2x89xa6xxe2x89xa61.3, 0.05xe2x89xa6y less than 0.9, 0.05xe2x89xa6z less than 0.9, 0.1xe2x89xa6y+zxe2x89xa60.9.
With the above composition, the positive electrode active material has stable crystal structure. The lithium rechargeable battery using this positive electrode active material has superior battery characteristics such as the cycle life characteristic and the storage characteristic. Another advantage in using this active material is that manganese, which is cheap and occurring in abundance, is used as an element of the active material.
Note that the composition of the lithium-containing metal oxide is analyzed using a known elementary analysis method, the inductively coupled plasma emission spectrometry.
In the above lithium rechargeable battery, when granular lithium-containing metal oxide is used, it is preferable that the average particle diameter of the lithium-containing metal oxide is in a range of 5 xcexcm to 25 xcexcm.
In the above lithium rechargeable battery, when granular lithium-containing metal oxide is used, it is preferable that the specific surface area on particles of the lithium-containing metal oxide is in a range of 0.1 m2/g to 5 m2/g.
Note that the average particle diameter is measured using the laser forward scattering method, and that the specific area is measured using the BET adsorption method.